1. Field of the Invention
This invention relates to laser markable articles, in particular to colour laser markable security documents.
2. Description of the Related Art
Security cards are widely used for various applications such as identification purposes (ID cards) and financial transfers (credit cards). Such cards typically consist of a laminated structure consisting of various paper or plastic laminates and layers wherein some of them may carry alphanumeric data and a picture of the card holder. So called ‘smart cards’ can also store digital information by including an electronic chip in the card body. A principal objective of such security cards is that they cannot be easily modified or reproduced in such a way that the modification or reproduction is difficult to distinguish from the original.
Two techniques frequently used for preparing security documents are laser marking and laser engraving. In literature, laser engraving is often incorrectly used for laser marking. In laser marking, a colour change is observed by local heating of material, while in laser engraving material is removed by laser ablation.
Well known in the field of laser markable security documents is the use of laser markable polymeric supports. Laser marking produces a colour change from white to black in a laser markable support through carbonization of the polymer, usually polycarbonate as disclosed in e.g. EP-A 2181858 (AGFA GEVAERT).
During the past last years, there is an increased interest of using laser markable layers. The advantage of using a laser markable layer coated on a support instead of a laser markable support, is that a support can be used which has better physical properties than the laser markable supports, such as for example a higher flexibility than a polycarbonate support as disclosed in e.g. EP-A 2567825 (AGFA GEVAERT).
There is also an increased interest in using laser marking to produce coloured images in a security document. Therefore, laser markable layers are used which are composed of colour forming compounds (also called “leuco-dyes”) which can change from essentially colourless or pale-coloured to coloured when exposed to for example heat, such as disclosed in for example EP-A 2648920.
The colour laser markable layers may comprise an infrared absorbing dye (IR dye) or an infrared absorbing pigment (IR pigment), both absorbing the IR radiation and converting it into heat.
An advantage of using IR dyes is that the absorption spectrum of an IR dye tends to be narrower than that of an IR pigment. This allows the production of multicoloured articles and security documents from precursors having a plurality of laser markable layers containing different IR dyes and colour foming compounds. The IR dyes having a different maximum absorption wavelength can then be addressed by IR lasers with corresponding emission wavelengths causing colour formation only in the laser markable layer of the addressed IR dye. Such multicolour articles has been disclosed in for example U.S. Pat. No. 4,720,449 and EP-A 2719540.
A problem however when using such an IR dye in a colour laser markable layer is often a non-linear response of the obtained colour density as function of the exposure energy. This may result in an insufficient reproduction of details of a colour image, especially in the highlights, i.e. in the low densities of that image.